Air cleaning apparatus

ABSTRACT

An air cleaning apparatus, especially for cleaning of room air, includes an ionizing device (14) having a unipolar ion source formed by a corona discharge electrode, an electrostatic precipitator (12) connected to a high-voltage source (15) and having a flow-through passageway (18) for air to be cleaned and two groups of electrode elements (16, 17) disposed in the flow-through passageway, the electrode elements (16) of one group being interleaved with and spaced from the electrode elements (17) of the other group and arranged to be at a potential different from that of the said other group. The corona discharge electrode (21) is arranged such that the ions generated at the electrode can diffuse essentially freely away from the electrode and thereby diffuse substantially freely throughout the room in which the ionizing device (14) is positioned.

BACKGROUND OF THE INVENTION

The present invention relates to an air cleaning apparatus, especiallyan apparatus for cleaning of room air. Room air air in dwelling rooms,office or other work-rooms, meeting rooms, etc., or in other words,rooms in the ordinary sense of the word. Room air also refers to air inother spaces, such as a space defined by a ventilation duct, which isseparated from the open air and thus holds a well-defined body of air.

More particularly, the invention relates to an air cleaning apparatuswhich comprises an ionizing device having a unipolar ion source formedby a corona discharge electrode, and an electrostatic precipitatorconnected to a high-voltage source and having a flow-through passagewayfor the air to be cleaned and two groups of electrode elements disposedin the flow-through passageway, the electrode elements of one groupbeing interleaved with and spaced from the electrode elements of theother group and arranged to be at a potential different from that of thesaid other group.

Today, air cleaning apparatus of this kind exist in the form oftwo-stage electrostatic filters in which the ionizing device comprises acorona discharge electrode in an ionizing chamber at the upstream orinlet side of the precipitator. The walls of the ionizing chamberenclose a well-defined space in which the charging of the dust particlestakes place more or less simultaneously with the entry of the air intothe precipitator. This space may be considered as an upstream extensionof the flow-through passageway of the precipitator, because all air thatpasses through the ionizing chamber and flows past the corona dischargeelectrode, flows on through the precipitator.

The efficiency of air cleaning apparatus of this kind is dependent onthe efficiency of the charging of the dust particles which the airflowthrough the ionizing chamber carries and which are to be separated fromthe air in the precipitator.

One way of achieving an efficient charging of the dust particles is topass a strong corona through the corona discharge electrode, but astrong corona current has the undesirable effect of causing asubstantial generation of ozone at the corona discharge electrode.

It is also possible to achieve an efficient charging of the dustparticles with a weak corona current by designing the ionization chambersuch that the dimension of the space charge region which is defined bythe ionizing chamber and through which the airborne particles passduring their travel to the precipitator is large in the direction offlow so that the particles will have a long dwelling time in this regionand, consequently, the time available for charging of the particles willalso be long. WO93/16807 discloses a two-stage electrostatic filter inwhich this technique of achieving an efficient charging of the dustparticles is embodied. Using the two stage electrostatic filterdescribed in this publication it is possible to achieve a dustseparation which is greatly improved over that of other prior arttwo-stage electrostatic filters, even though the corona current is veryweak.

SUMMARY OF THE INVENTION

The present invention provides a radically different way of achieving,in an air cleaning apparatus of the kind initially defined, an efficientseparation of dust particles from the air using a very weak coronacurrent.

In practicing the present invention, it has been found that even whenthe ionizing device is supplied with a very weak corona current, it ispossible to achieve a charging of the airborne dust particles that issufficiently strong to enable an efficient separation in anelectrostatic precipitator, without using an ionization chamber in whichthe dust particles are subjected to an "intensive treatment" shortlybefore they enter into the precipitator.

Although the air cleaning apparatus according to the invention comprisea unipolar ion source formed by a corona discharge electrode, it has noionization chamber in the ordinary sense of the word. Instead, thecorona discharge electrode is arranged such that the ions produced atthe electrode may diffuse away from the electrode substantially freelyand fill the entire space, such as a room in a building or a section ofa ventilation duct, in which the body of air to be cleaned is enclosed.

In other words, in the air cleaning apparatus according to theinvention, the corona electrode is not associated with diffusionbarriers or screens that prevent or substantially oppose the freediffusion of the ion cloud in the space, that is, the room in which theionizer is placed.

The corona discharge electrode may be located practically anywhere inthe room. Accordingly, it is basically possible to use as the ionizingdevice a so-called ionizer of the type available on the market--it mayproduce positive or negative ions--and place it practically anywhere inthe room, regardless of the position of the precipitator. Provided thatthere are no barriers or screens of the kind mentioned, the cloud ofions produced at the corona discharge electrode will diffuse throughoutthe room, that is throughout the body of air to be cleaned, and chargethe dust particles in it.

However, it may be preferable that the ionizing device is joined withthe precipitator, e.g. with the corona discharge electrode mounted onthe exterior surface of a housing which enclosed the precipitator andpreferably also a fan or other device for transporting air through theprecipitator, as well as the required high-voltage source is common tothe corona discharge electrode and the precipitator.

Although the housing may itself form a barrier which prevents diffusionof the ion cloud in one direction, straight rearwardly, the ion cloudcan still diffuse in the opposite direction, towardly, and laterally andalso rearwardly by the sides of the housing, so that it will ultimatelyfill the entire room.

Accordingly, it is sufficient that there is a region which is free fromdiffusion barriers at least in one direction and permits the ion cloudto spread freely, so that the ion cloud can reach all parts of the roomin which the ionizing device is placed.

It is possible, therefore, to position the corona discharge electrode inan inset, and thus protected, position in the housing, provided ofcourse that the space in the apparatus in which the electrode ispositioned is open outwardly such that the outward ion diffusion is notsubstantially inhibited.

It is preferred to use as the precipitator a capacitor precipitator of adesign that permits efficient separation even of particles which are notstrongly charged, because the more efficient the separation is, theweaker the corona current can be without sacrificing the cleaningefficiency.

A requirement for an efficient separation is that the electric fieldstrength between adjacent electrode element of the precipitator isalways as high as possible, that is, it should always be so high thatthe precipitator always operates at or only slightly below the fieldstrength at which sparkover occurs. It therefore is advantageous to usea capacitor precipitator of the kind described in WO88/03057 andWO93/16807.

In capacitor precipitators of this kind, the voltage between adjacentelectrode elements is automatically and individually, independently ofthe other electrode element pairs, adjusted to be at or close to thevoltage limit of the high-voltage source. Such precipitators can also beadvantageously designed as inexpensive and environment-friendlydisposable products so that they may be discarded when they have becomeso loaded with dust that they no longer can effect an adequateseparation.

If in such a capacitor precipitator the sparkover limit should beexceeded for an electrode element pair, e.g. as a consequence of dustaccumulation on one of the electrode elements of the pair, a discharge(glow discharge) will occur so that a current will flow between theelectrode elements, but this current will be very weak because of thehigh resistivity of the electrode elements and any existing separatecurrent-limiting resistors of the high-voltage source.

At most, the discharge will cause the voltage between the electrodeelements of the pair to drop somewhat, namely to a value just aboutsufficient to maintain the discharge. The electrode elements maytherefore still effect precipitation of the charged particles passingbetween them, albeit at a reduced efficiency because of the loweredvoltage. The other electrode element pairs are not affected by thedischarge.

Primarily, the invention is suitable for application to air cleaningapparatus designed for airflow rates of 100 cubic metres or more,preferably substantially more, per hour.

BRIEF DESCRIPTION OF THE DRAWING

This invention will be described in greater detail below with referenceto the accompanying drawing, the single FIGURE of which shows alongitudinal sectional view of an air cleaning apparatus embodying theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The air cleaning apparatus shown in the drawing comprises a housing orenclosure 11 which encloses a readily replaceable electrostatic dustprecipitator, generally designated by 12, and an air transporting devicein the form of an electrically driven fan 13, which transports airthrough the apparatus such that the air passes through a flow-throughpassageway in the dust precipitator. An arrow P indicated the directionof flow of the air, and as is evident from the drawing, the dustseparator 12 is positioned upstream of the fan 13.

Associated with the air cleaning apparatus is an ionizing device, namelya so-called ionizer, generally designated by 14, which may be mounted onthe housing 11 and thus supported by the housing, but it may also bedetached and standing by itself and thus have no mechanical connectionwith the housing. In operation, the ionizer 14 preferably is positionedin front of the air inlet of the housing 11, or at least near the inlet.

Moreover, the air cleaning apparatus comprises a high-voltage source(direct current source) which is generally designated by 15 in thedrawing and which is common to the dust precipitator 12 and the ionizer14.

The dust precipitator 12 is a capacitor precipitator having twointerleaved groups of parallel, preferably plate-like electrode elements16 and 17 defining between them a large number of subpassageways 18which jointly form the above-mentioned flow-through passageway for theair to be cleaned. Via a high-ohmic resistor 19 the electrode elements16 of one group are connected to the negative terminal of thehigh-voltage source 15, and the electrode element 17 of the other groupare similarly connected to the positive terminal via a high-ohmicresistor.

Advantageously, the dust precipitator is constructed as described inWO88/03057 so that at least the electrode elements 17, and preferablyalso the electrode elements 16, are made of a semiconducting, so-calledantistatic or dissipative material, or coated with such a material.Moreover, the connection of at least the electrode elements 17 andsuitably also the electrode elements 16, connected to the high-voltagesource 15 is accomplished via, in addition to the respective resistors20 and 19, a strip or bar shaped contact member (not shown) ofhigh-ohmic material.

Alternatively, the dust precipitator 12 may be constructed as describedin WO93/16807, the electrode elements in one group, namely the groundedelectrode elements, being kept at the required potential with the aid ofa separate corona discharge device.

It is also preferable to provide the edges of the electrode elementsmade of or coated with a semiconducting or antistatic material, or atleast that edge, the upstream or leading edge, which is directed againstthe direction of flow of the air, with a screen of an electricallyconducting material as is described in PCT/SE94/00131.

Regardless of which of the above-mentioned constructions is chosen forthe dust precipitator 12, the dust precipitator suitably is made from ahigh-ohmic fibrous material, preferably a cellulose material, such aspaper or cardboard.

Within the scope of the invention, the ionizer 14 may be of any suitabletype having a corona discharge electrode as a unipolar, positive ornegative, ion source, such as an ionizer of any of the commerciallyavailable types and constructions.

The ionizer 14 shown only by way of example in the drawing comprises acorona discharge electrode 21 in the shape of a wire or point electrode,which is connected to the positive terminal of the high voltage source15. It is supported by a holder 22 of an insulating material andencircled by an annular conducting excitation or counter electrode 23connected to the grounded negative terminal of the high-voltage source.Suitably, the distance between the point of the corona dischargeelectrode 21 and the adjacent portion of the excitation electrode 23 isgreater than 3 cm.

Excitation electrode 23 may also take many other forms than that shownin the drawing. For example, it may be a grounded grid or other groundedair permeable member mounted over the inlet opening of the dustprecipitator. If the ionizer comprises a separate excitation electrodecorresponding to that which is shown in the drawing, an inlet grid orthe like made of plastic may be positioned over the inlet opening.

It is not necessary, but preferable, to use an ionizer comprising aseparate excitation electrode as shown in the drawing. For example, theelectrode elements of the dust precipitator may serve as the excitationelectrode when the ionizer is positioned in front of the dustprecipitator at the inlet to the housing 11 of the air cleaningapparatus.

Within the scope of the invention, other modifications of theillustrated embodiments are possible. For example, the corona dischargeelectrode 21 may be a wire loop or a linear wire electrode.

Moreover, the ionizer 14 may be positioned substantially as desired,although for practical reasons it is preferred that it forms part of theunit comprising the housing 11, the dust precipitator 12, the fan 13 andthe high voltage source 15. One case in which it may be preferred tohave the ionizer separated from the rest of the air cleaning apparatusis when the housing 11 with the dust precipitator 12 is inserted in aventilation duct or the like or for some reason is not suitably placedin the room the air of which is to be cleaned.

Naturally, the ionizer must be positioned such that the ion cloud itgenerates is not inhibited by screening elements or barriers of somekind from diffusing essentially freely in the room containing the air tobe cleaned.

We claim:
 1. Air cleaning apparatus comprising, in combination:anionizing device (14) having a unipolar ion source formed by a coronadischarge electrode (21) arranged such that ions generated at theelectrode can diffuse in at least one direction essentially freely anduninhibited by any barrier away from the electrode into the spacecontaining the air to be cleaned, An electrostatic precipitator (12)connected to a high-voltage source (15), said electrostatic precipitator(12) defining a flow-through passageway (18) for air to be cleaned andincluding first and second groups or electrode elements (16, 17)disposed in the flow-through passageway, the electrode elements (16) ofsaid first group being interleaved with and spaced from the electrodeelements (17) of the second group and arranged to be at a potentialdifferent from that of said second group, said electrode elements (16,17) of at least one of said first and second groups of electrodeelements being made from or coated with a high-ohmic material, whereinthe entire corona discharge electrode is axially spaced from theelectrostatic precipitator.
 2. Air cleaning apparatus according to claim1, wherein the corona discharge electrode (21) is supported by a housing(11) which encloses the electrostatic precipitator (12).
 3. Air cleaningapparatus according to claim 2, wherein the corona discharge electrode(21) is disposed adjacent an inlet to the flow-through passageway (18).4. Air cleaning apparatus according to claim 2, wherein the coronadischarge electrode (21) is disposed on an exterior wall of the housing(11).
 5. Air cleaning apparatus according to any one of claims 1 to 4,wherein the electrode elements of at least one of said first and secondgroups of electrode elements (16, 17) are made of a fibrous material. 6.Air cleaning apparatus according to claim 5, wherein the electrodeelements of at least one of said first and second groups of electrodeelements (16, 17) have an individual, high-ohmic connection with thehigh-voltage source (15).
 7. Air cleaning apparatus according to claim1, wherein the corona discharge electrode (21) and the electrostaticprecipitator (12) are each connected to said high-voltage source (15).8. Air cleaning apparatus according to claim 7, wherein theelectrostatic precipitator (12) forms a unit with an air transportingdevice (13) for transporting air through the flow-through passageway(18) of the precipitator (12).
 9. An air cleaning apparatuscomprising:an ionizing device having a unipolar ion source formed by acorona discharge electrode arranged such that ions generated at theelectrode can diffuse in at least one direction essentially freely anduninhibited by any barrier away from the electrode into the spacecontaining the air to be cleaned, an electrostatic precipitatorconnected to a high-voltage source, said electrostatic precipitatordefining a flow-through passageway for air to be cleaned and includingfirst and second groups of electrode elements disposed in theflow-through passageway, the electrode elements of said first groupbeing interleaved with and spaced from the electrode elements of thesecond group and arranged to be at a potential different from that ofsaid second group, said electrode elements of at least one of said firstand second groups of electrode elements being made from or coated with ahigh-ohmic material, and a housing enclosing the electrostaticprecipitator, and wherein the entire corona discharge electrode isdisposed exterior to the housing.
 10. The air cleaning apparatus ofclaim 9, wherein the corona discharge electrode is disposed adjacent aninlet to the flow-through passageway.
 11. The air cleaning apparatus ofclaim 9, wherein the electrode elements of at least one of said firstand second groups of electrode elements is made of a fibrous material.12. The air cleaning apparatus of claim 9, wherein the corona dischargeelectrode is mounted on an exterior surface of the housing.
 13. An aircleaning apparatus comprising:an electrostatic precipitator connected toa high-voltage source, said electrostatic precipitator defining aflow-through passageway for air to be cleaned and including first andsecond groups of electrode elements disposed in the flow-throughpassageway, the electrode elements of said first group being interleavedwith an spaced from the electrode elements of the second group andarranged to be at a potential different from that of said second group,said electrode elements of at least one of said first and second groupsof electrode elements being made from or coated with a high-ohmicmaterial; an ionizing device axially spaced from the electrostaticprecipitator, said ionizing device including a corona dischargeelectrode supported by an insulating holder surrounded by a conductiveexcitation electrode, said corona discharge electrode being connected toa positive terminal of said high voltage source, and said excitationelectrode being connected to a grounded negative terminal of saidhigh-voltage source, said corona discharge electrode being arranged suchthat ions generated at the electrode can diffuse essentially freely awayfrom the electrode; and a fan for transporting air through theflow-through passageway of the precipitator, said fan being disposeddownstream of the electrostatic precipitator.
 14. The air cleaningapparatus of claim 13, wherein the corona discharge electrode isdisposed exterior to the housing.
 15. The air cleaning apparatus ofclaim 14, wherein the corona discharge electrode is disposed adjacent aninlet to the flow-through passageway.
 16. The air cleaning apparatus ofclaim 13, wherein the electrode elements of at least one of said firstand second groups of electrode elements is made of a fibrous material.